Hibernate查询语言(HQL)是一种面向对象的查询语言,类似于SQL,但不是对表和列操作,HQL适用于持久对象和它们的属性。 HQL查询由Hibernate转换成传统的SQL查询,这在圈上的数据库执行操作。
虽然可以直接使用SQL语句和Hibernate使用原生SQL,但建议使用HQL尽可能避免数据库可移植性的麻烦,并采取Hibernate的SQL生成和缓存策略的优势。
都像SELECT,FROM和WHERE等关键字不区分大小写,但如表名和列名的属性是区分在HQL敏感。
FROM 语句
使用FROM子句,如果要加载一个完整的持久化对象到内存中。下面是一个使用FROM子句的简单的语法:
1
2
3
|
String hql = "FROM Employee" ; Query query = session.createQuery(hql); List results = query.list(); |
1
2
3
|
String hql = "FROM com.hibernatebook.criteria.Employee" ; Query query = session.createQuery(hql); List results = query.list(); |
AS 语句
AS子句可以用来别名分配给类中的HQL查询,特别是当有很长的查询。例如,我们前面简单的例子是以下几点:
1
2
3
|
String hql = "FROM Employee AS E" ; Query query = session.createQuery(hql); List results = query.list(); |
AS关键字是可选的,也可以直接在之后的类名指定别名,如下所示:
1
2
3
|
String hql = "FROM Employee E" ; Query query = session.createQuery(hql); List results = query.list(); |
SELECT 子句
SELECT子句提供了更多的控制权比from子句的结果集。如果想获得对象而不是整个对象的几个属性,使用SELECT子句。下面是一个使用SELECT语句来获取Employee对象只是FIRST_NAME字段的简单的语法:
1
2
3
|
String hql = "SELECT E.firstName FROM Employee E" ; Query query = session.createQuery(hql); List results = query.list(); |
值得注意的是在这里,Employee.firstName是Employee对象的一个属性,而不是EMPLOYEE表的一个字段。
WHERE 子句
如果想缩小了从存储返回的特定对象,可以使用WHERE子句。下面是一个使用WHERE子句的简单的语法:
1
2
3
|
String hql = "FROM Employee E WHERE E.id = 10" ; Query query = session.createQuery(hql); List results = query.list(); |
ORDER BY 子句
若要排序HQL查询的结果,将需要使用ORDER BY子句。您可以在结果集按升序(ASC)或降序(DESC)通过在对象的任何属性排序结果。下面是一个使用ORDER BY子句的简单的语法:
1
2
3
|
String hql = "FROM Employee E WHERE E.id > 10 ORDER BY E.salary DESC" ; Query query = session.createQuery(hql); List results = query.list(); |
如果想通过一个以上的属性进行排序,你会仅仅是额外的属性添加到由子句用逗号隔开,如下所示的命令的结尾:
1
2
3
4
|
String hql = "FROM Employee E WHERE E.id > 10 " + "ORDER BY E.firstName DESC, E.salary DESC " ; Query query = session.createQuery(hql); List results = query.list(); |
GROUP BY 子句
该子句允许从Hibernate的它基于属性的值的数据库和组提取信息,并且通常使用结果包括总值。下面是一个使用GROUP BY子句的语法很简单:
1
2
3
4
|
String hql = "SELECT SUM(E.salary), E.firtName FROM Employee E " + "GROUP BY E.firstName" ; Query query = session.createQuery(hql); List results = query.list(); |
使用命名参数
Hibernate命名在其HQL查询参数支持。这使得编写接受来自用户的输入容易,不必对SQL注入攻击防御HQL查询。下面是一个使用命名参数的简单的语法:
1
2
3
4
|
String hql = "FROM Employee E WHERE E.id = :employee_id" ; Query query = session.createQuery(hql); query.setParameter( "employee_id" , 10 ); List results = query.list(); |
UPDATE 子句
批量更新是新的HQL与Hibernate3,以及不同的删除工作,在Hibernate 3和Hibernate2一样。 Query接口现在包含一个名为executeUpdate()方法用于执行HQL UPDATE或DELETE语句。
在UPDATE子句可以用于更新一个或多个对象中的一个或多个属性。下面是一个使用UPDATE子句的简单的语法:
1
2
3
4
5
6
7
|
String hql = "UPDATE Employee set salary = :salary " + "WHERE id = :employee_id" ; Query query = session.createQuery(hql); query.setParameter( "salary" , 1000 ); query.setParameter( "employee_id" , 10 ); int result = query.executeUpdate(); System.out.println( "Rows affected: " + result); |
DELETE 子句
DELETE子句可以用来删除一个或多个对象。下面是一个使用DELETE子句的简单的语法:
1
2
3
4
5
6
|
String hql = "DELETE FROM Employee " + "WHERE id = :employee_id" ; Query query = session.createQuery(hql); query.setParameter( "employee_id" , 10 ); int result = query.executeUpdate(); System.out.println( "Rows affected: " + result); |
INSERT 子句
HQL支持INSERT INTO子句中只记录在那里可以插入从一个对象到另一个对象。以下是使用INSERT INTO子句的简单的语法:
1
2
3
4
5
|
String hql = "INSERT INTO Employee(firstName, lastName, salary)" + "SELECT firstName, lastName, salary FROM old_employee" ; Query query = session.createQuery(hql); int result = query.executeUpdate(); System.out.println( "Rows affected: " + result); |
聚合方法
HQL支持多种聚合方法,类似于SQL。他们工作在HQL同样的方式在SQL和下面的可用功能列表:
DISTINCT关键字只计算在该行设定的唯一值。下面的查询将只返回唯一的计数:
1
2
3
|
String hql = "SELECT count(distinct E.firstName) FROM Employee E" ; Query query = session.createQuery(hql); List results = query.list(); |
使用查询分页
有用于分页查询接口的两个方法。
- Query setFirstResult(int startPosition)
- Query setMaxResults(int maxResult)
采用上述两种方法一起,可以在网站或Swing应用程序构建一个分页组件。下面是例子,可以扩展来获取10行:
1
2
3
4
5
|
String hql = "FROM Employee" ; Query query = session.createQuery(hql); query.setFirstResult( 1 ); query.setMaxResults( 10 ); List results = query.list(); |
查询条件
Hibernate提供了操作对象,并依次数据在RDBMS表可用的备用方式。其中一个方法是标准的API,它允许你建立一个标准的查询对象编程,可以套用过滤规则和逻辑条件。
Hibernate的Session接口提供了可用于创建一个返回的持久化对象的类的实例时,应用程序执行一个条件查询一个Criteria对象createCriteria()方法。
以下是最简单的一个条件查询的例子是将简单地返回对应于Employee类的每个对象。
1
2
|
Criteria cr = session.createCriteria(Employee. class ); List results = cr.list(); |
限制与标准:
可以使用add()方法可用于Criteria对象添加限制条件查询。下面是例子增加一个限制与薪水返回的记录是等于2000:
1
2
3
|
Criteria cr = session.createCriteria(Employee. class ); cr.add(Restrictions.eq( "salary" , 2000 )); List results = cr.list(); |
以下是几个例子覆盖不同的场景,并且可以根据要求使用:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
|
Criteria cr = session.createCriteria(Employee. class ); // To get records having salary more than 2000 cr.add(Restrictions.gt( "salary" , 2000 )); // To get records having salary less than 2000 cr.add(Restrictions.lt( "salary" , 2000 )); // To get records having fistName starting with zara cr.add(Restrictions.like( "firstName" , "zara%" )); // Case sensitive form of the above restriction. cr.add(Restrictions.ilike( "firstName" , "zara%" )); // To get records having salary in between 1000 and 2000 cr.add(Restrictions.between( "salary" , 1000 , 2000 )); // To check if the given property is null cr.add(Restrictions.isNull( "salary" )); // To check if the given property is not null cr.add(Restrictions.isNotNull( "salary" )); // To check if the given property is empty cr.add(Restrictions.isEmpty( "salary" )); // To check if the given property is not empty cr.add(Restrictions.isNotEmpty( "salary" )); 可以创建AND或OR使用LogicalExpression限制如下条件: Criteria cr = session.createCriteria(Employee. class ); Criterion salary = Restrictions.gt( "salary" , 2000 ); Criterion name = Restrictions.ilike( "firstNname" , "zara%" ); // To get records matching with OR condistions LogicalExpression orExp = Restrictions.or(salary, name); cr.add( orExp ); // To get records matching with AND condistions LogicalExpression andExp = Restrictions.and(salary, name); cr.add( andExp ); List results = cr.list(); |
虽然上述所有条件,可以直接使用HQL在前面的教程中介绍。
分页使用标准:
还有的标准接口,用于分页的两种方法。
- public Criteria setFirstResult(int firstResult)
- public Criteria setMaxResults(int maxResults)
采用上述两种方法一起,我们可以在我们的网站或Swing应用程序构建一个分页组件。下面是例子,可以扩展来每次获取10行:
1
2
3
4
|
Criteria cr = session.createCriteria(Employee. class ); cr.setFirstResult( 1 ); cr.setMaxResults( 10 ); List results = cr.list(); |
排序的结果:
标准的API提供了org.hibernate.criterion.Order类排序按升序或降序排列你的结果集,根据对象的属性。这个例子演示了如何使用Order类的结果集进行排序:
1
2
3
4
5
6
7
8
9
10
11
|
Criteria cr = session.createCriteria(Employee. class ); // To get records having salary more than 2000 cr.add(Restrictions.gt( "salary" , 2000 )); // To sort records in descening order crit.addOrder(Order.desc( "salary" )); // To sort records in ascending order crit.addOrder(Order.asc( "salary" )); List results = cr.list(); |
预测与聚合:
该Criteria API提供了一个org.hibernate.criterion.Projections类可用于获取平均值,最大值或最小值的属性值。Projections类是类似于类限制,因为它提供了几个静态工厂方法用于获得Projection 实例。 provides the
以下是涉及不同的方案的一些例子,可按规定使用:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
|
Criteria cr = session.createCriteria(Employee. class ); // To get total row count. cr.setProjection(Projections.rowCount()); // To get average of a property. cr.setProjection(Projections.avg( "salary" )); // To get distinct count of a property. cr.setProjection(Projections.countDistinct( "firstName" )); // To get maximum of a property. cr.setProjection(Projections.max( "salary" )); // To get minimum of a property. cr.setProjection(Projections.min( "salary" )); // To get sum of a property. cr.setProjection(Projections.sum( "salary" )); |
Criteria Queries 例子:
考虑下面的POJO类:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
|
public class Employee { private int id; private String firstName; private String lastName; private int salary; public Employee() {} public Employee(String fname, String lname, int salary) { this .firstName = fname; this .lastName = lname; this .salary = salary; } public int getId() { return id; } public void setId( int id ) { this .id = id; } public String getFirstName() { return firstName; } public void setFirstName( String first_name ) { this .firstName = first_name; } public String getLastName() { return lastName; } public void setLastName( String last_name ) { this .lastName = last_name; } public int getSalary() { return salary; } public void setSalary( int salary ) { this .salary = salary; } } |
让我们创建下面的EMPLOYEE表来存储Employee对象:
1
2
3
4
5
6
7
|
create table EMPLOYEE ( id INT NOT NULL auto_increment, first_name VARCHAR (20) default NULL , last_name VARCHAR (20) default NULL , salary INT default NULL , PRIMARY KEY (id) ); |
以下将被映射文件。
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
|
<? xml version = "1.0" encoding = "utf-8" ?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"> < hibernate-mapping > < class name = "Employee" table = "EMPLOYEE" > < meta attribute = "class-description" > This class contains the employee detail. </ meta > < id name = "id" type = "int" column = "id" > < generator class = "native" /> </ id > < property name = "firstName" column = "first_name" type = "string" /> < property name = "lastName" column = "last_name" type = "string" /> < property name = "salary" column = "salary" type = "int" /> </ class > </ hibernate-mapping > |
最后,我们将创建应用程序类的main()方法来运行,我们将使用Criteria查询的应用程序:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
|
import java.util.List; import java.util.Date; import java.util.Iterator; import org.hibernate.HibernateException; import org.hibernate.Session; import org.hibernate.Transaction; import org.hibernate.SessionFactory; import org.hibernate.Criteria; import org.hibernate.criterion.Restrictions; import org.hibernate.criterion.Projections; import org.hibernate.cfg.Configuration; public class ManageEmployee { private static SessionFactory factory; public static void main(String[] args) { try { factory = new Configuration().configure().buildSessionFactory(); } catch (Throwable ex) { System.err.println( "Failed to create sessionFactory object." + ex); throw new ExceptionInInitializerError(ex); } ManageEmployee ME = new ManageEmployee(); /* Add few employee records in database */ Integer empID1 = ME.addEmployee("Zara", "Ali", 2000); Integer empID2 = ME.addEmployee("Daisy", "Das", 5000); Integer empID3 = ME.addEmployee("John", "Paul", 5000); Integer empID4 = ME.addEmployee("Mohd", "Yasee", 3000); /* List down all the employees */ ME.listEmployees(); /* Print Total employee's count */ ME.countEmployee(); /* Print Toatl salary */ ME.totalSalary(); } /* Method to CREATE an employee in the database */ public Integer addEmployee(String fname, String lname, int salary){ Session session = factory.openSession(); Transaction tx = null; Integer employeeID = null; try{ tx = session.beginTransaction(); Employee employee = new Employee(fname, lname, salary); employeeID = (Integer) session.save(employee); tx.commit(); }catch (HibernateException e) { if (tx!=null) tx.rollback(); e.printStackTrace(); }finally { session.close(); } return employeeID; } /* Method to READ all the employees having salary more than 2000 */ public void listEmployees( ){ Session session = factory.openSession(); Transaction tx = null; try{ tx = session.beginTransaction(); Criteria cr = session.createCriteria(Employee.class); // Add restriction. cr.add(Restrictions.gt("salary", 2000)); List employees = cr.list(); for (Iterator iterator = employees.iterator(); iterator.hasNext();){ Employee employee = (Employee) iterator.next(); System.out.print("First Name: " + employee.getFirstName()); System.out.print(" Last Name: " + employee.getLastName()); System.out.println(" Salary: " + employee.getSalary()); } tx.commit(); }catch (HibernateException e) { if (tx!=null) tx.rollback(); e.printStackTrace(); }finally { session.close(); } } /* Method to print total number of records */ public void countEmployee(){ Session session = factory.openSession(); Transaction tx = null; try{ tx = session.beginTransaction(); Criteria cr = session.createCriteria(Employee.class); // To get total row count. cr.setProjection(Projections.rowCount()); List rowCount = cr.list(); System.out.println("Total Coint: " + rowCount.get(0) ); tx.commit(); }catch (HibernateException e) { if (tx!=null) tx.rollback(); e.printStackTrace(); }finally { session.close(); } } /* Method to print sum of salaries */ public void totalSalary(){ Session session = factory.openSession(); Transaction tx = null ; try { tx = session.beginTransaction(); Criteria cr = session.createCriteria(Employee. class ); // To get total salary. cr.setProjection(Projections.sum( "salary" )); List totalSalary = cr.list(); System.out.println( "Total Salary: " + totalSalary.get( 0 ) ); tx.commit(); } catch (HibernateException e) { if (tx!= null ) tx.rollback(); e.printStackTrace(); } finally { session.close(); } } } |
编译和执行:
下面是步骤来编译并运行上述应用程序。请确保您已在进行的编译和执行之前,适当地设置PATH和CLASSPATH。
- 创建hibernate.cfg.xml配置文件中配置章节解释。
- 创建Employee.hbm.xml映射文件,如上图所示。
- 创建Employee.java源文件,如上图所示,并编译它。
- 创建ManageEmployee.java源文件,如上图所示,并编译它。
- 执行ManageEmployee二进制运行程序.
会得到以下结果,并记录将创建在EMPLOYEE表中。
1
|
$java ManageEmployee |
1
2
3
4
5
6
7
|
.......VARIOUS LOG MESSAGES WILL DISPLAY HERE........ First Name: Daisy Last Name: Das Salary: 5000 First Name: John Last Name: Paul Salary: 5000 First Name: Mohd Last Name: Yasee Salary: 3000 Total Coint: 4 Total Salary: 15000 |
如果检查EMPLOYEE表,它应该记录如下:
1
|
mysql> select * from EMPLOYEE; |
1
2
3
4
5
6
7
8
9
|
+----+------------+-----------+--------+ | id | first_name | last_name | salary | +----+------------+-----------+--------+ | 14 | Zara | Ali | 2000 | | 15 | Daisy | Das | 5000 | | 16 | John | Paul | 5000 | | 17 | Mohd | Yasee | 3000 | +----+------------+-----------+--------+ 4 rows in set (0.00 sec) |